Power line communication (herein abbreviated PLC) refers to systems for enabling data to be transferred over electrical cables. PLC is also referred to in the art as a power line digital subscriber line, a power line carrier, mains communication, power line telecom and power line networking. Electrical cables can also be referred to as power cables, power lines, electrical power lines, electrical wiring, electrical cabling and the like. These terms are used interchangeably herein and represent the cabling used to transfer electricity from an electricity provider, such as an electric company (e.g. Pacific Gas & Electric, Florida Power & Light, etc. . . . ) or an electricity generator (e.g., a wind energy converter), to a residence, as well as the wires used in a residence to transfer electricity to various wall sockets, electrical outlets, wall plugs and power points in the residence.
PLC enables various electrical devices, such as computers, printers, televisions and other electrical devices in a residence, to be coupled with one another as a network without the need for new wires to be added to the residence. A residence can refer to a private home, an apartment building, an office building or other structures where people live that receive electricity. In effect, the electric cabling forms the backbone of a power line network or a PLC network. Each electrical device to be coupled in the network requires a separate communication device for enabling it to transfer data over the electrical wiring. Such a communication device is usually referred to as a modem, and commonly referred to in the art as a power line modem. Such modems usually transfer data in a high frequency range, such as on the order of megahertz or higher. PLC systems and methods are known in the art.
Traditionally, power lines and their associated networks were designed for providing electricity and not for the purposes of communication and were thus not designed to provide an optimal medium for transferring data. Power line networks suffer from high levels of noise, which distorts and interferes with communication signals. Noise in PLC networks can be defined as any undesirable voltage signal which travels along the power line network and which might be received as a communication signal in one of the power line modems coupled with the network. Common sources of noise are various household devices coupled to the power line network.
Reference is now made to FIG. 1A, which is a schematic illustrations of a prior art system, generally referenced 10, for coupling a PLC communication device to a PLC network in a residence. With reference to FIG. 1A, coupling system 10 includes a PLC device 12 and a transformer 14. PLC device 12 may be a PLC modem. Transformer 14 inductively couples PLC device 12 to the PLC network (not shown). In particular a modem first line 16 and a modem second line 18 are coupled with a first winding (not referenced) of transformer 14. Network phase line 20 and network neutral line 22 are coupled with a second winding (not referenced) of transformer 14. Network phase line 20 refers to the phase line (or active line) in the residence, whereas network neutral line 22 refers to the neutral line in the residence. Together, network phase line 20 and network neutral line 22 define a network phase-neutral (herein abbreviated PN) interface (not referenced). Modem first line 16 and modem second line 18 define a modem PN interface (not referenced), which is inductively coupled with the network PN interface through transformer 14.
The noise in PLC networks can be classified into two main categories, common mode (herein abbreviated CM) noise and differential mode (herein abbreviated DM) noise. CM noise is a signal which is referenced to the ground wire in a PLC network and which is injected simultaneously with the same polarity to two different lines in a PLC network. Hence, CM noise can affect two or more elements of a PLC network in a similar manner. DM noise is a signal which is injected simultaneously with opposing polarities to two different lines in a PLC network. Models are known in the art for modeling CM noise and DM noise in PLC networks, as shown in FIGS. 1B and 1C respectively. In FIG. 1B, CM noise is modeled and filtered out by the transformer. In FIG. 1C, DM noise is modeled and is not filtered out by the transformer.
Reference is now made to FIGS. 1B and 1C, which are schematic illustrations of noise models in PLC networks, generally referenced 10′ and 10″, as is known in the prior art. It is noted that equivalent elements in FIGS. 1A-1C are referenced using identical numbering. With reference to FIG. 1B, coupling system 10′ includes all the elements of the coupling system shown in FIG. 1A. Coupling system 10′ further includes an equivalent CM noise voltage source 24 and a ground terminal 26 for modeling the interaction of CM noise with a PLC network (not shown) on the network PN interface. Voltage source 24 is coupled with both network phase line 20 and with network neutral line 22. Voltage source 24 produces CM noise signals on both network phase line 20 and on network neutral line 22. In the ideal case, this results in zero CM noise signals on the modem PN interface, as the noise is filtered out by transformer 14 on the modem PN interface side (not referenced).
A balanced interface is an interface consisting of two similar ports (or lines), each having substantially similar impedance relative to ground (i.e., ground impedance). For example, in FIG. 1B, the network PN interface is, in theory, a balanced interface as the CM noise signals produced by voltage source 24 cancel out each other at transformer 14 and are not reflected to PLC device 12. It is noted that CM noise signals are often produced by household devices coupled with the power line network or are produced internally by devices of the PLC network, such as the power supply (not shown) of PLC device 12, which is coupled with the primary winding (not referenced) of transformer 14.
With reference to FIG. 1C, coupling system 10″ includes all the elements of the coupling system shown in FIG. 1A. Coupling system 10″ also includes a pair of voltage sources 28 and 30 and a ground terminal 32 for modeling the interaction of DM noise with a PLC network (not shown). Voltage source 28 is coupled between ground terminal 32 and network phase line 20. Voltage source 30 is coupled between ground terminal 32 and network neutral line 22. Pair of voltage sources 28 and 30 are similar in power but are opposite in polarity, as is shown in FIG. 1C (i.e., the polarity of voltage source 28 is opposite that of voltage source 30). Pair of voltage sources 28 and 30 produce a DM noise signal on the network PN interface. In particular, voltage source 28 produces a first portion of the DM noise signal on network phase line 20. Voltage source 30 produces a second portion of the DM noise signal, which is opposite in amplitude to the first portion of the DM noise signal, on network neutral line 22. Transformer 14 induces the DM noise signal into PLC device 12. Thus, the DM noise signal is not filtered out by transformer 14. It is noted that the main source for DM noise signals in a PLC network is the communication signal itself. Additionally, other noise sources in the electrical system of the residence may also generate a DM noise component.
Reference is now made to FIG. 2, which is a schematic illustration of a coupling system, generally referenced 50, for inductively coupling a communication device to a power line network, as is known in the art. Coupling system 50 includes a communication device 52, a first transformer 60 and a second transformer 62. Communication device 52 will be referred to herein as modem 52. The communication section (not referenced) of modem 52 is coupled with first transformer 60 and second transformer 62. It is noted that even though modem 52 is employed as both a transmitter and a receiver for the electrical device, the example set forth with reference to FIG. 2 details the receiver functionality of modem 52. The communication section of modem 52 is coupled with a first winding 70 of first transformer 60 (i.e., a modem side winding) through a first modem line 54 and a second modem line 56. The communication section of modem 52 is also coupled with a first winding 72 of second transformer 62 through a third modem line 57 and a fourth modem line 58.
A phase line 64 and a neutral line 66 of the PLC network (not referenced) are coupled with a second winding 74 of first transformer 60 (i.e., a network side winding). Each of phase line 64 and neutral line 66 includes a respective capacitor 65A and 65B for safety purposes. Neutral line 66 and a ground line 68 of the PLC network are coupled with a second winding 76 of second transformer 62. Phase line 64 and neutral line 66 define a network PN interface (not referenced). Neutral line 66 and ground line 68 define a network ground-neutral (herein abbreviated NG) interface (not referenced). First modem line 54 and second modem line 56 together define a modem PN interface, which is inductively coupled with the network PN interface through first transformer 60. Third modem line 57 and fourth modem line 58 define a modem NG interface, which is inductively coupled with the network NG interface through second transformer 62.
Phase line 64 and neutral line 66 are employed for delivering power through the power line network. Phase line 64 is also referred to as an active line or a live line. Ground line 68 is employed for safety purposes. Coupling system 50 inductively couples modem 52 to the power line network through first and second transformers 60 and 62 respectively. Modem 52 is a communication device for transmitting and receiving communication signals to and from other communication devices in the PLC network, such as other PLC modems (not shown) coupled with other electrical devices (not shown) in the residence. For example, a remote PLC modem (not shown) transmits a modulated signal through the PLC network and specifically through coupling system 50 to modem 52. In a similar manner, modem 52 can transmit a modulated signal to the remote PLC modem through coupling system 50 and through the PLC network.
As can be seen in FIG. 2, the network PN interface is not balanced. Put another way, the ground impedance of phase line 64 is different than that of neutral line 66, since the ground impedance of phase line 64 includes a summation of the impedances of both first transformer 60 and second transformer 62 whereas the ground impedance of neutral line 66 includes only the impedance of second transformer 62. It is noted that the impedance of each of first transformer 60 and second transformer 62 is dependent at least on the impedance of the modem PN interface and the modem NG interface, respectively. Due to the lack of symmetry between the ground impedance of phase line 64 and of neutral line 66, CM noise signals (not shown) on the network PN interface do not fully cancel each other out on first transformer 60.